The present invention generally relates to photoplethysmographic measurement systems and, in particular, to an electrical lockout for avoiding undesired transmission of optical signals by such photoplethysmographic measurement systems.
In the field of photoplethysmography, light pulses from different portions of the electromagnetic spectrum are used to noninvasively measure various blood analytes or blood oxygen saturation in a test subject. The photoplethysmography measurement system generally includes a main instrument, a removable probe for attaching to a patient during a test, and a signal transmitter for delivering signals between the main instrument and the probe. Because the probe ordinarily makes direct contact with the patient""s body during a medical examination, it is desirable that probes be disposable in order to reduce the risk of infection. Accordingly, the probe is typically detachable from the main instrument via a coupling disposed somewhere between the probe and housing.
To reduce cost and to make the removable probe head affordable as a disposable unit, the componentry of the probe head is minimized by separating the light sources from the probe head to the main instrument or to another portion of the system (such as within a connector extending from the housing to the probe) that is detachably coupled to the probe head. While such disposable probe heads are relatively inexpensive to manufacture since the expensive light source elements and processing circuitry are located in the main instrument, the presently known photoplethysmography measurement devices do pose certain shortcomings. The light output from the fiber optic cable in conventional systems typically remains continuously turned on as long as the main instrument remains turned on. Thus, when the probe head becomes detached while the main instrument is active, the intense light emission from the end of the fiber optic cable could potentially expose a patient or clinician to intense illumination.
The inventor has recognized that the operation of a photoplethysmographic measurement system may be further enhanced by incorporating a safety feature to effectively prevent users, such as patients and medical personnel, from being exposed to high levels of light energy from the system when a probe becomes detached from the system during use. If the main instrument is able to sense when the probe becomes detached therefrom, certain operations may be performed therein in order to avoid energizing, or to de-energize, the light sources upon receiving an indication that the probe detached therefrom.
According to one aspect of the invention, any appropriate means for sensing detachment of the probe from the main instrument may be used. The present invention may employ inherent features of the probe and the coupling device in order to detect detachment of the probe. The coupling device between the main instrument and probe generally includes optical and electrical connections. For purposes of sensing detachment, the main instrument may be configured to detect breach of either the optical or the electrical connection. Alternatively, new features may be added to the system for purposes of providing feedback. For example, a resistor or other electrical component may be incorporated into the probe so as to provide an electronic signature or a feedback. In this case, the main instrument has a feedback detector for detecting such an electrical component in the probe when it is properly connected thereto. Furthermore, the present invention may use a dedicated sensor in order to provide indication of proper connection. For example, the connector port may be provided with a contact for purposes of indicating connection with the probe. Alternatively, an optical sensor may be employed to provide indication of connection across coupling sections.
According to the invention, a photoplethysmographic measurement system for noninvasively measuring various blood analytes in a patient""s appendage is provided. The measurement system includes an electrical lockout which frequently or continuously interrogates the probe connection and automatically disconnects current flow to the light emitters upon receiving an indication that the probe or the cable is detached. The probe may include one or more preselected electrical elements which can be detected by the lockout of the main instrument. For purposes of determining if the probe is properly connected or has been accidentally disconnected, the electrical lockout frequently or continuously monitors an absence or presence of such electrical element.
In one embodiment, the probe is provided with a resistor having a predetermined resistor value. In use, the electrical lockout continuously interrogates the probe connection in order to determine whether the value of the detected resistor falls within predetermined allowable levels. If the probe accidentally becomes disconnected from the main instrument, the detected value of the resistor will fall outside of the predetermined allowable levels indicative of an incompatible probe, an improper coupling of the probe, or a complete detachment of the probe. In this case, the lockout switch is employed to inhibit activation of the light emitters. When a proper connection is re-established, the electrical lockout will recognize the value of the detected resistor and send a signal to close the lockout switch, allowing current to pass and re-energize the light sources.
In this embodiment, the value of the resistor also serves to identify the type of probe presently connected to the system. A probe may be uniquely designed for attaching to a given appendage of a patient, such as a finger, earlobe, or the nasal septum so as to accommodate a variety of monitoring, conditions and situations. For purposes of obtaining the most accurate measurement, the identified probe-type information may be employed to determine the appropriate input light signals to be used to illuminate the patient""s appendage and the appropriate computation to be used to process the transmitted light signals.
In another embodiment, the electrical lockout may include means for detecting the presence of a light sensor in the probe by checking for the appropriate forward voltage drop on the conductors connected to the light sensor.
In another embodiment, the probe is provided with a bandgap reference, wherein the amplitude of the bandgap reference voltage generated within the probe will be used to determine the presence and identify the type of probe. In this case, the electrical lockout includes a voltage detector to interrogate for a voltage drop at certain allowable levels at the probe head. If the bandgap reference voltage falls outside of the predetermined allowable voltage levels, the lockout will send a signal to open a lockout switch to turn off the light emitters.
In yet another embodiment, a contact, or set of contacts in the probe are required to continue to power the light emitters. Thus disconnecting the probe removes power (or turns off a control signal) to the light emitters thus turning off the light emitters.
Finally, any of these techniques may be used in combination to eliminate the possibility of emitters staying on in the presence of a single point failure.